Optical recording medium inspecting method and optical recording medium manufacturing method

ABSTRACT

It is an object of the present invention to provide an inspection method for judging whether or not a phase change type medium can be used as a write-once type medium.  
     In the present invention, shortest signals are recorded in an optical recording medium having a phase change type recording layer so that the CNR of the shortest signal is equal to or higher than 45 dB, an area in which the shortest signal was recorded is irradiated with a constant-current laser beam whose power level cannot melt the recording layer at the same linear speed as that at which the shortest signal was recorded, and decrease in the carrier of the shortest signal is measured. When the decrease in the carrier of the shortest signal is equal to or lower than 20 dB, the optical recording medium is judged to be a write-once type medium.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method for inspecting and a method for manufacturing an optical recording medium having a phase change type recording layer and adapted to be used as a write-once type medium.

DESCRIPTION OF THE PRIOR ART

[0002] In recent years, optical recording media which can record data with high density have attracted attention. These optical recording media include write-once type media in which data can be recorded only once and data cannot be rewritten, and rewritable type media in which data can be repeatedly recorded.

[0003] Since recorded data cannot be rewritten in the write-once type media, the write-once type media are suitable for recording official documents and the like whose contents are not allowed to be rewritten. As write-once type media, write-once type media using an organic dye as a recording material are widely used. However, in the case where an organic dye is employed as a recording material, since recording sensitivity tends to be not high when high speed recording is conducted by increasing the linear speed of the medium, high transmission rate cannot be achieved. Further, since the spectral absorption characteristic and the spectral reflection characteristic of the organic dye are relatively steep, it is necessary to select an organic dye matched to the wavelengths for recording and reproducing data. Therefore, for example, when there is a higher level format using light of a short wavelength for recording and reproducing data, it is impossible to record data in and reproduce data from a medium of a lower level using light for recording data in and reproducing data from the higher level format. Moreover, it is difficult to design or obtain an organic dye matched to light of a short wavelength for recording and reproducing data.

[0004] On the other hand, among rewritable type optical recording media, in the phase change type, data are recorded by irradiating a recording layer with a laser beam, thereby changing the crystal state of the recording layer and data are reproduced by detecting change in reflectance of the recording layer caused by such change in the crystal state. In the phase change type media in which data can be rewritten by overwriting them, an amorphous record mark is formed by irradiating a crystalline recording layer with a laser beam having a recording power level, melting it and quickly cooling the melted portion of the recording layer. When data are to be erased, an amorphous record mark is crystallized by irradiating it with a laser beam having an erasing power level, heating the amorphous record mark to a temperature equal to or higher than the crystallization temperature and lower than the melting point of the recording layer and gradually cooling it. Therefore, it is possible to overwrite data by irradiating the recording layer with a laser beam whose power is being modulated.

[0005] The media having a phase change type recording layer can be used not only as the above-mentioned rewritable media but also as write-once type media. When the media are used as write-once type media, it is required that a once formed amorphous record mark be impossible to erase or rewrite.

[0006] In write-once type media using an organic dye, the organic dye is decomposed during data recording. As a consequence, it is generally said that when the linear speed for recording data is doubled, it is necessary to make the power of the laser beam for recording to 2^(1/2) times. On the other hand, in the case where phase change type media are used as write-once type media, it is sufficient for the temperature of a portion irradiated with the laser beam for recording to reach the melting point thereof. Since the recording layer absorbs the laser beam and the temperature thereof reaches the melting point instantaneously, the power of the laser beam for recording does not greatly depend upon the recording linear speed. Therefore, even if the linear speed for recording data is doubled, it is sufficient to slightly increase the power of the laser beam for recording.

[0007] However, no useful idea has been proposed for using phase change type media as write-once type media.

[0008] It is an object of the present invention to provide an inspection method for judging whether or not an optical recording medium can be effectively used as a write-once type medium. Moreover, it is also an object of the present invention to provide a method for manufacturing an optical recording medium having a phase change type recording layer and adapted to be used as a write-once type medium.

SUMMARY OF THE INVENTION

[0009] In order to use a phase change type medium which has been developed as a rewritable type medium as a write-once type medium, the phase change medium has to have a characteristic of not allowing new data to be substantially overwritten on once recorded data. Generally, in a phase change type medium, severer conditions are required for overwriting new data on once recorded data than those required for newly writing data in an unrecorded area. Therefore, in order to use a phase change type medium as a write-once type medium, it is necessary that data can be newly written in an unrecorded area under predetermined recording conditions while data cannot be newly overwritten on once recorded data.

[0010] It is an object of the present invention to provide an inspection method for judging whether or not an optical recording medium can be effectively used as a write-once type medium and the object of the present invention can be accomplished by a method for inspecting an optical recording medium comprising the steps of recording a shortest signal in an optical recording medium having a phase change type recording layer so that CNR of the shortest signal is equal to or higher than 45 dB, irradiating an area in which the shortest signal was recorded with a constant-current laser beam whose power level cannot melt the recording layer at the same linear speed as that at which the shortest signal was recorded, measuring decrease in the carrier of the shortest signal and judging the optical recording medium to be a write-once type medium when the decrease in the carrier is equal to or lower than 20 dB.

[0011] The above objects of the present invention can be also accomplished by a method for inspecting an optical recording medium comprising the steps of recording random signals in the optical recording medium having a phase change type recording layer, conducting reproducing operation of the signals on an area in which the random signals were recorded, thereby measuring a highest reflective level Rini, irradiating the area in which the random signals were recorded with a constant-current laser beam at the same linear speed as that at which the random signals were recorded, conducting reproducing operation of the signals on the area irradiated with the constant-current laser beam, thereby measuring a highest reflective level Rtop and a lowest reflective level Rbottom, and judging the optical recording medium to be a write-once type medium when (Rtop+Rbottom)/2Rini<1 is satisfied, irrespective of whether the constant-current laser beam has a power level which can melt the recording layer.

[0012] The above objects of the present invention can be also accomplished by a method for inspecting an optical recording medium comprising the steps of recording random signals in the optical recording medium having a phase change type recording layer, again recording the random signals so as to be superposed on the first mentioned random signals at the same linear speed as that at which the first mentioned random signals were recorded, conducting reproducing operation of the signals, and judging the optical recording medium to be a write-once type medium when the signals cannot be reproduced.

[0013] The above objects of the present invention can be also accomplished by a method for inspecting an optical recording medium comprising the steps of recording random signals in the optical recording medium having a phase change type recording layer, irradiating an area in which the random signals were recorded with a constant-current laser beam at the same linear speed as that at which the random signals were recorded, thereby again recording random signals in the area irradiated with the constant-current laser beam, conducting reproducing operation of the signals, and judging the optical recording medium to be a write-once type medium when the signals cannot be reproduced.

[0014] According to each of the methods for inspecting optical recording media, it is possible to effectively judge whether or not an optical recording medium to be inspected is constituted so that data can be newly rewritten in an unrecorded area under predetermined recording conditions and that new data cannot be overwritten on once recorded data.

[0015] The above objects of the present invention can be also accomplished by a method for manufacturing an optical recording medium comprising a layer forming step of forming at least a phase change recording layer, an initialization step of crystallizing at least an area of the recording layer in which data are to be recorded and an inspection step of judging whether the optical recording medium subjected to the initialization step is a write-once type medium or a rewritable type medium, the inspecting step comprising the steps of recording a shortest signal in an optical recording medium having a phase change type recording layer so that CNR of the shortest signal is equal to or higher than 45 dB, irradiating an area in which the shortest signal was recorded with a constant-current laser beam whose power level cannot melt the recording layer at the same linear speed as that at which the shortest signal was recorded, measuring decrease in the carrier of the shortest signal and judging the optical recording medium to be a write-once type medium when the decrease in the carrier is equal to or lower than 20 dB.

[0016] The above objects of the present invention can be also accomplished by a method for manufacturing an optical recording medium comprising a layer forming step of forming at least a phase change recording layer, an initialization step of crystallizing at least an area of the recording layer in which data are to be recorded and an inspection step of judging whether the optical recording medium subjected to the initialization step is a write-once type medium or a rewritable type medium, the inspecting step comprising the steps of recording random signals in the optical recording medium having a phase change type recording layer, conducting reproducing operation of the signals on an area in which the random signals were recorded, thereby measuring a highest reflective level Rini, irradiating the area in which the random signals were recorded with a constant-current laser beam at the same linear speed as that at which the random signals were recorded, conducting reproducing operation of the signals on the area irradiated with the constant-current laser beam, thereby measuring a highest reflective level Rtop and a lowest reflective level Rbottom, and judging the optical recording medium to be a write-once type medium when (Rtop+Rbottom)/2Rini<1 is satisfied, irrespective of whether the constant-current laser beam has a power level which can melt the recording layer.

[0017] The above objects of the present invention can be also accomplished by a method for manufacturing an optical recording medium comprising a layer forming step of forming at least a phase change recording layer, an initialization step of crystallizing at least an area of the recording layer in which data are to be recorded and an inspection step of judging whether the optical recording medium subjected to the initialization step is a write-once type medium or a rewritable type medium, the inspecting step comprising the steps of recording random signals in the optical recording medium having a phase change type recording layer, again recording the random signals so as to be superposed on the first mentioned random signals at the same linear speed as that at which the first mentioned random signals were recorded, conducting reproducing operation of the signals, and judging the optical recording medium to be a write-once type medium when the signals cannot be reproduced.

[0018] The above objects of the present invention can be also accomplished by a method for manufacturing an optical recording medium comprising a layer forming step of forming at least a phase change recording layer, an initialization step of crystallizing at least an area of the recording layer in which data are to be recorded and an inspection step of judging whether the optical recording medium subjected to the initialization step is a write-once type medium or a rewritable type medium, the inspecting step comprising the steps of recording random signals in the optical recording medium having a phase change type recording layer, irradiating an area in which the random signals were recorded with a constant-current laser beam at the same linear speed as that at which the random signals were recorded, thereby again recording random signals in the area irradiated with the constant-current laser beam, conducting reproducing operation of the signals, and judging the optical recording medium to be a write-once type medium when the signals cannot be reproduced.

[0019] According to each of the methods for manufacturing optical recording media, it is possible to manufacture an optical recording medium constituted so that data can be newly rewritten in an unrecorded area under predetermined recording conditions and that new data cannot be overwritten on once recorded data.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a flowchart showing an inspection method for judging whether or not a first condition which a phase change type medium to be used as a write-once type medium should meet is satisfied.

[0021]FIG. 2 is a flowchart showing an inspection method for judging whether or not a second condition which a phase change type medium to be used as a write-once type medium should meet is satisfied.

[0022]FIG. 3 is a flowchart showing an inspection method for judging whether or not a third condition which a phase change type medium to be used as a write-once type medium should meet is satisfied.

[0023]FIG. 4 is a flowchart showing an inspection method for judging whether or not a fourth condition which a phase change type medium to be used as a write-once type medium should meet is satisfied.

[0024]FIG. 5 is a partial cross-sectional view showing one example of an optical recording medium which is used in the present invention.

[0025]FIG. 6 is a partial cross-sectional view showing another example of an optical recording medium which is used in the present invention.

[0026]FIG. 7 is a graph showing a 5T signal and its record waveform.

[0027]FIG. 8 is a photograph substituted for a drawing showing crystal structure and is a transmission electron microscope photograph of a recording layer.

[0028]FIG. 9 is a graph showing a relationship between a DC erase power and (Rtop+Rbottom)/2Rini.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Preferred embodiments of the present invention will be explained with reference to the drawings.

[0030] Regardless of whether a phase change type medium is a rewritable type or a write-once type, it is required to have improved recording density. However, when the length of a record mark is shortened in order to record data with high density, reproduction output tends to decrease and jitter tends to increase. On the other hand, Japanese Patent Application Laid Open No. 2000-231725 proposes to mitigate decrease in reproduction output and increase in jitter caused by data recording with high density by controlling the shapes of record marks. Japanese Patent Application Laid Open No. 2000-231725 discloses an optical recording method for forming a shortest record mark having a shape at least a part of whose rear end portion is projected toward a front end portion, namely, a bat-like shape. Such a bat-like shortest record mark can be formed by controlling recording conditions. A recording layer melted by a laser beam for recording is quickly cooled as the laser beam is moved away, thereby forming an amorphous record mark. At this time, if the cooling speed in the vicinity of the rear end portion of the melted area is controlled by controlling the power modulation pattern of the laser beam, the rear half of the melted area can be again crystallized. As a result, only the front half of the melted area becomes amorphous and a bat-like amorphous record mark can be formed.

[0031] According to the method disclosed in this publication, the width of the record mark can be increased with respect to the length thereof and, therefore, the reduction of the reproduction output caused by narrowing of the record mark width can be suppressed. As a result, according to the method disclosed in this publication, even when the length of the shortest record mark is set to be equal to or shorter than 0.4λ/NA wherein λ is the wavelength of recording light and NA is the numeral aperture of an objective lens of the recording optical system, sufficient record mark width can be ensured, whereby sufficient reproduction output can be obtained. Moreover, in this publication, an attempt is made to reduce jitter by forming the shortest record mark so as to have the above described shape.

[0032] On the other hand, in the phase change type medium, severer conditions are required for overwriting new data on the once recorded data than those required for newly writing data in an unrecorded area. Therefore, it is effective to employ a recording layer whose crystallization rate is high in order to overwrite data at a high recording linear speed. This means that in order to use a phase change type medium as a write-once type medium, it is preferable to employ a recording layer whose crystallization rate is relatively low and set a linear speed to be relatively high so that new data can be written and that data cannot be overwritten.

[0033] Similarly to the case of recording data in the rewritable medium described in the Working Example of Japanese Patent Application Laid Open No. 2000-231725, the inventors of the present invention experimented with recording data in a phase change type medium which was constituted in order to be used as a write-once type medium so as to have a recording layer whose crystallization rate was relatively low and to be adapted for a relatively high recording linear speed in such a manner that the length of the shortest record mark was set to be equal to or shorter than 0.4λ/NA and that the rear portion of a melted area would be again crystallized. However, in this case, jitter was increased, unlike the case of the rewritable medium described in the Working Example of Japanese Patent Application Laid Open No. 2000-231725.

[0034] It can be considered that the reason jitter was increased when data were recorded in the write-once type medium with high density was as follows. In the experiment, a recording layer whose crystallization rate was relatively low was provided and the recording linear speed was set to be relatively high in order to use the phase change type medium as a write-once type medium. However, in the case where the shortest record mark having a length equal to or shorter than 0.4λ/NA was formed under these conditions so as to again crystallize the rear portion of a melted area, since the crystallization rate of the recording layer was relatively low and the melted recording layer resisted being crystallized again, the position of the rear end portion of the record mark was apt to fluctuate and the shape of the record mark was apt to vary. As a result, it can be considered that jitter was increased at the shortest record mark dominantly affecting jitter of the whole medium.

[0035] Based on the results of this experiment, the inventors of the present invention tried to prevent the shape of record marks from varying by recording data under the condition where the rear portion of a melted area of a recording layer was suppressed. Concretely, the crystallization rate of the recording layer and the heat design of the medium were controlled in accordance with the recording linear speed and optimum recording conditions were set up so that high recording sensitivity could be ensured and the shortest record mark was formed so as to have a circular shape or an elliptical shape. However, if the shortest record mark is formed to as to have a circular shape or an elliptical shape, the reproduction output is lowered as in the Comparative Example of Japanese Patent Application Laid Open No. 2000-231725. Therefore, in the present invention, the recording layer is selected so as to have such a composition that the crystallization rate is relatively low and that change in the reflection coefficient is large between the crystal state and the amorphous state. As a result, according to the present invention, it is possible to provide such a phase change type medium provided with a recording layer having a crystallization rate usable for a write-once type medium that jitter is small and a high reproduction output can be obtained when data are recorded with high density.

[0036] In the present invention, the statement that a high reproduction output can be obtained when data are recorded with high density means that the CNR (carrier to noise ratio) of the shortest record mark is equal to or higher than 45 dB, preferably 48 dB when the length of the shortest record mark is equal to or shorter than 0.4λ/NA.

[0037] Further, in the present invention, there are the following first to fourth conditions which a phase change type medium used as a write-once type medium should meet. At least one of the first to fourth conditions has to be met in order to use a phase change type medium as a write-once type medium.

[0038] The first condition is that reduction in carrier of the shortest signal is equal to or lower than 20 dB, preferably 18 dB, in the case of recording data in an optical recording medium having a phase change type recording layer so that the CNR of the shortest signal is equal to or higher than 45 dB, preferably 48 dB, and conducting an erasing operation by irradiating the optical recording medium with a laser beam having a power level which cannot melt the recording layer at the same linear speed as that at which data were recorded. If the reduction in carrier lies within this range, it is impossible to read signals recorded after the erasing operation. It was already known to use a phase change type medium as a write-once type medium but conditions necessary for using a phase change type medium as a write-once type medium were not clear. In a rewritable phase change type medium, it was known that if the erasure coefficient is equal to or higher than 25 dB, data could be again recorded after erasing data, namely, data could be rewritten. Therefore, it can be assumed that data cannot be rewritten, namely, recorded data cannot be interpolated if the erasure coefficient is lower than 25 dB. However, in a study made by the inventors of the present invention, it has been found that signals recorded after the erasing operation can be read even if the erasure coefficient is lower than 25 dB, unless the reduction in carrier of the shortest signal is equal to or lower than 20 dB.

[0039] Although it is necessary to form amorphous record marks in a crystalline recording layer in the present invention, in the case where a phase change type recording layer is formed by a gas phase growth process such as a sputtering, the recording layer is formed to be amorphous. Therefore, it is necessary to crystallize at least an area of the recording layer in which data are to be recorded prior to recording data. This crystallization is normally referred to as initialization. However, it is extremely difficult to crystallize the recording layer immediately after it was formed. Therefore, if the optical recording medium is designed in such a manner that reduction in carrier of the shortest signal is too small, namely, the recording layer is extremely resistant to being crystallized again, the initialization has to be conducted at an extremely low linear speed and the productivity of the optical recording media is inevitably lowered. Considering this, in the first condition, the reduction in carrier of the shortest signal caused by the erasing operation is preferably equal to or higher than 5 dB and when a recording linear rate is relatively low, it is preferably equal to or higher than 10 dB.

[0040]FIG. 1 is a flowchart showing an inspection method for judging whether or not the first condition is met.

[0041] When inspection is to be conducted for judging whether or not the first condition is met, an optical recording medium to be inspected is first set in an inspection apparatus (not shown) at Step S1. It is necessary for the inspection apparatus to be constituted at least so as to be able to record arbitrary signals (record marks) in an optical recording medium under arbitrary recording conditions such as laser beam power, recording linear speed and the like, and measure the carrier levels of the record marks recorded in the optical recording medium and the reflection coefficients. However, this does not mean that the inspection has to be carried out using a single inspection apparatus and the inspection may be carried out using a plurality of apparatuses.

[0042] A “shortest signal” is then set for the inspection apparatus as a recording signal to be recorded in the optical recording medium at Step S2, and a predetermined laser beam power and a predetermined recording linear speed are further set as recording conditions at Step S3.

[0043] When the setup has been completed, a signal sequence consisting of the shortest signals is actually recorded in the optical recording medium to be inspected at Step S4 and the CNR thereof is measured at Step S5. The CNR obtained at Step S5 is defined as a CNR1.

[0044] It is then judged at Step S6 whether or not the thus measured CNR is equal to or higher than a predetermined value and when it is judged that the CNR is lower than the predetermined value, the inspection operation is returned to Step 3 and the recording conditions are reset. The predetermined value has to be determined to be equal to or higher than 45 dB, preferably 48 dB.

[0045] When it is judged at Step 6 that the CNR is equal to or higher than the predetermined value, reading operation is effected on the optical recording medium and the reflective levels thereof are measured at Step 7. The highest reflective level obtained at Step S7 is defined as an Rtop1.

[0046] A “constant-current laser beam” is then set for the inspection apparatus as a recording signal at Step S8 and recording conditions are set at Step 9 in such a manner that the recording linear speed is set to be the same as that set at Step 6 and that the power of the constant-current laser beam is changed step by step. It is necessary to set the interval between successive changes in the power of the constant-current laser beam so as to be sufficiently longer than a clock cycle and, for example, the power of the constant-current laser beam may be stepwise changed once each time the optical recording medium makes one revolution. When the setup has been completed, the constant-current laser beam whose power is controlled so as to be changed step by step is at Step 10 projected onto the shortest signal sequence recorded at Step S4.

[0047] The reading operation is again effected on the optical recording medium to be inspected and reflective levels of respective areas irradiated with the constant-current laser beams having different powers are measured at Step S11. The highest reflective level obtained at Step S11 is defined as an Rtop2.

[0048] It is then judged at Step S12 by comparing the reflective level Rtop1 obtained at Step 7 and the reflective level Rtop2 obtained at Step S11 whether or not the reflective level Rtop2 is substantially lower than the reflective level Rtopl. When it is found that the reflective level Rtop2 is substantially lower than the reflective level Rtop1, it is judged that the recording layer has been melted by the constant-current laser beam projected at Step S8 and the inspection operation is returned to Step S4, thereby recording the signal sequence consisting of the shortest signals in a new area of the recording layer.

[0049] On the other hand, when it is found that the reflective level Rtop2 is not substantially lower than the reflective level Rtop1, it is judged that the recording layer was not melted by the constant-current laser beam projected at Step S8 and the CNR is measured at Step 13. The CNR obtained at Step S13 is defined as CNR2.

[0050] It is then judged at Step 14 whether or not the difference (CNR1−CNR2) between the CNR1 obtained at Step 5 and the CNR2 obtained in Step S13 is equal to or smaller than a predetermined value. It is preferable to set the predetermined value to be equal to or smaller than 20 dB, preferably equal to or smaller than 18 dB.

[0051] When it is found that the difference (CNR1−CNR2) between the CNR1 and the CNR2 is equal to or smaller than the predetermined value, it is judged at Step S15 that the optical recording medium is a write-once type medium. To the contrary, when it is found that the difference between the CNR1 and the CNR2 exceeds the predetermined value, it is judged at Step S16 that the optical recording medium is a rewritable type medium.

[0052] From the viewpoint of productivity of the optical recording medium, in the case where it is judged at Step S16 that the optical recording medium is a write-once type medium, it is preferable to further determine whether or not the difference (CNR1−CNR2) between the CNR1 and the CNR2 is equal to or larger than 5 dB, preferably 10 dB, and judge that the optical recording medium is not suitable for a write-once type medium or a rewritable type medium when it is found that the difference between the CNR1 and the CNR2 is less than 5 dB or 10 dB.

[0053] Thus, it is possible to judge whether or not the optical recording medium meets the first condition.

[0054] With respect to the second condition, in the case of recording random signals in an optical recording medium having a phase change type recording layer, conducting reproducing operation of the signals on the area in which the random signals were recorded, thereby measuring a highest reflective level Rini, irradiating the area in which the random signals were recorded with a constant-current laser beam at the same linear speed as that at which the random signals were recorded, thereby conducting erasing operation of the signals, and conducting reproducing operation of the signals on the area irradiated with the constant-current laser beam, thereby measuring a highest reflective level Rtop and a lowest reflective level Rbottom, when (Rtop+Rbottom)/2Rini is less than 1, preferably equal to or less than 0.95, the second condition is met. The reflective level is defined as an amount of light returning to an optical head and the highest reflective level Rini is a reflective level of crystalline areas each disposed between neighboring record marks.

[0055] The second condition has to be met irrespective of the power level of the constant-current laser beam. Specifically, (Rtop+Rbottom)/2Rini has to be within the above defined range irrespective of whether the constant-current laser beam has a power level which can melt the recording layer. In the case where the constant-current laser beam does not melt the recording layer, if the second condition is met, it is impossible to crystallize the recording layer in solid phase by the irradiation with the constant-current laser beam. On the other hand, in the case where the constant-current laser beam melts the recording layer, if the second condition is met, it is possible to cool a melted recording layer so that the recording layer can be made amorphous. To the contrary, when a rewritable type medium is irradiated with a constant-current laser beam while the power level thereof is being gradually increased, since the recording layer is crystallized in solid phase or from liquid phase, there can be a power level of the constant-current laser beam so that (Rtop+Rbottom)/2Rini is equal to or larger than 1.

[0056] It is preferable to record the random signals under an optimum recording condition. In this case, the optimum recording condition is selected from a condition which can make jitter of signals recorded in the recording layer immediately after the initialization thereunder, an optimum recording condition recommended by a recording medium manufacturer or an optimum recording condition defined by a standard for the optical recording medium.

[0057]FIG. 2 is a flowchart showing an inspection method for judging whether or not the second condition is met.

[0058] When inspection is to be conducted for judging whether or not the second condition is met, an optical recording medium to be inspected is first set in an inspection apparatus (not shown) at Step S21. It is necessary for the inspection apparatus to be constituted at least so as to be able to record arbitrary signals (record marks) in an optical recording medium under arbitrary recording conditions such as laser beam power, recording linear speed and the like, and measure jitter of the record marks recorded in the optical recording medium and the reflection coefficients. However, this does not mean that the inspection has to be carried out using a single inspection apparatus and the inspection may be carried out using a plurality of apparatuses.

[0059] A “random signal” is then set for the inspection apparatus as a recording signal to be recorded in the optical recording medium at Step S22, and a predetermined laser beam power and a predetermined recording linear speed are further set as recording conditions at Step S23.

[0060] When the setup has been completed, a signal sequence consisting of the random signals is actually recorded in the optical recording medium to be inspected at Step S24 and the jitter thereof is measured at Step S25.

[0061] It is then judged at Step 26 whether the thus measured jitter is equal to or lower than a predetermined value. When it is found that the jitter exceeds the predetermined value, the inspection operation is returned to Step 23 and the recording conditions are reset. The predetermined value is preferably set to be equal to or lower than 9%, preferably 8%.

[0062] When it is judged at Step 26 that the jitter is equal to or lower than the predetermined value, reading operation is effected on the optical recording medium and the reflective levels thereof are measured at Step 27. The highest reflective level obtained at Step S27 is defined as an Rini.

[0063] A “constant-current laser beam” is then set for the inspection apparatus as a recording signal at Step S28 and recording conditions are set at Step 29 in such a manner that the recording linear speed is set to be the same as that set at Step 23 and that the power of the constant-current laser beam is changed step by step. It is necessary to set the interval between successive changes in the power of the constant-current laser beam so as to be sufficiently longer than a clock cycle and, for example, the power of the constant-current laser beam may be stepwise changed once each time the optical recording medium makes one revolution.

[0064] When the setup has been completed, the constant-current laser beam whose power is controlled so as to be changed step by step is projected at Step 10 onto the random signal sequence recorded at Step S24.

[0065] The reading operation is again effected on the optical recording medium to be inspected and reflective levels of respective areas irradiated with the constant-current laser beams having different powers are measured at Step S31. The highest reflective level obtained at Step S31 is defined as an Rtop and the lowest reflective level obtained at Step S31 is defined as an Rbottom.

[0066] It is then judged at Step 32 whether or not the Rini obtained at Step 27 and the Rtop and the Rbottom obtained at Step 31 meet a predetermined condition that (Rtop +Rbottom)/2Rini is less than 1, preferably meet a predetermined condition that (Rtop +Rbottom)/2Rini is equal to or less than 0.95.

[0067] When it is found at Step 32 that the predetermined condition is satisfied, it is judged at Step 33 that the optical recording medium is a write-once type medium. To the contrary, when it is found at Step 32 that the predetermined condition is satisfied, it is judged at Step S34 that the optical recording medium is a rewritable type medium.

[0068] Thus, it is possible to judge whether or not the optical recording medium meets the second condition.

[0069] The third condition is that reading of signals cannot be effected in the case of recording random signals in an optical recording medium having a phase change type recording layer, recording random signals on the thus recorded random signals at the same rate as that at which the first mentioned random signals were recorded and reading the signals. The random signals are preferably recorded under an optimum recording condition. In this case, the optimum recording condition is selected from a condition which can make jitter of signals recorded in the recording layer immediately after the initialization thereunder, an optimum recording condition recommended by a recording medium manufacturer or an optimum recording condition defined by a standard for the optical recording medium.

[0070] In the present invention, the statement that reading of signals cannot be effected means that reproduced signal contains error which cannot be corrected, preferably means that clock jitter of the reproduced signal is larger than 13% and more preferably means that the clock jitter is larger than 15%.

[0071]FIG. 3 is a flowchart showing an inspection method for judging whether or not the third condition is met.

[0072] When inspection is to be conducted for judging whether or not the third condition is met, an optical recording medium to be inspected is first set in an inspection apparatus (not shown) at Step S41. It is necessary for the inspection apparatus to be constituted at least so as to be able to record arbitrary signals (record marks) in an optical recording medium under arbitrary recording conditions such as laser beam power, recording linear speed and the like, and measure jitter of the record marks recorded in the optical recording medium and the reflection coefficients. However, this does not mean that the inspection has to be carried out using a single inspection apparatus and the inspection may be carried out using plurality of apparatuses.

[0073] A “random signal” is then set for the inspection apparatus as a recording signal to be recorded in the optical recording medium at Step S42, and a predetermined laser beam power and a predetermined recording linear speed are further set as recording conditions at Step S43.

[0074] When the setup has been completed, a signal sequence consisting of the random signals is actually recorded in the optical recording medium to be inspected at Step S44 and the jitter thereof is measured at Step S45.

[0075] It is then judged at Step 46 whether the thus measured jitter is equal to or lower than a predetermined value. When it is found that the jitter exceeds the predetermined value, the inspection operation is returned to Step 43 and the recording conditions are reset. The predetermined value is preferably set to be equal to or lower than 9%, preferably 8%.

[0076] When it is judged at Step 46 that the jitter is equal to or lower than the predetermined value, a random signal sequence is at Step 47 again recorded on the random signal sequence recorded at Step 44 under the same conditions and the jitter thereof is measured at Step 48.

[0077] It is then judged at Step 49 whether or not the jitter exceeds a level indicating that a reproduced signal contains error which cannot be corrected, concretely 13%, preferably 15%. When it is found that the jitter exceeds 13% or 15%, it is judged at Step 50 that the optical recording medium is a write-once type medium. To the contrary, when it is found that the jitter does not exceed 13% or 15%, it is judged at Step 51 that the optical recording medium is a rewritable type medium.

[0078] Thus, it is possible to judge whether or not the optical recording medium meets the third condition.

[0079] The fourth condition is that reading of signals cannot be effected in the case of recording random signals in an optical recording medium having a phase change type recording layer, irradiating an area in which the random signal were recorded with a constant-current laser beam at the same linear speed as that at which the random signal were recorded, thereby conducting an erasing operation, again recording random signals in the area irradiated with the constant-current laser beam and conducting a reading operation. The random signals are preferably recorded under the above described optimum recording conditions.

[0080] The fourth condition has to be met irrespective of the power level of the constant-current laser beam. Specifically, it is required that the random signals recorded after the irradiation with the constant-current laser beam cannot be read, irrespective of whether the constant-current laser beam has a power level which can melt the recording layer. In the case where the constant-current laser beam does not melt the recording layer, if the fourth condition is met, it is impossible to crystallize the recording layer in solid phase by the irradiation with the constant-current laser beam. On the other hand, in the case where the constant-current laser beam melts the recording layer, if the fourth condition is met, it is possible to cool a melted recording layer so that the recording layer can be made amorphous. To the contrary, in a rewritable type medium, since the recording layer is crystallized in solid phase or from liquid phase, data can be again recorded in the area irradiated with the constant-current laser beam in either case.

[0081]FIG. 4 is a flowchart showing an inspection method for judging whether or not the fourth condition is met.

[0082] When inspection is to be conducted for judging whether or not the fourth condition is met, an optical recording medium to be inspected is first set in an inspection apparatus (not shown) at Step S61. It is necessary for the inspection apparatus to be constituted at least so as to be able to record arbitrary signals (record marks) in an optical recording medium under arbitrary recording conditions such as laser beam power, recording linear speed and the like, and measure jitter of the record marks recorded in the optical recording medium and the reflection coefficients. However, this does not mean that the inspection has to be carried out using a single inspection apparatus and the inspection may be carried out using a plurality of apparatuses.

[0083] A “random signal” is then set for the inspection apparatus as a recording signal to be recorded in the optical recording medium at Step S62, and a predetermined laser beam power and a predetermined recording linear speed are further set as recording conditions at Step S63.

[0084] When the setup has been completed, a signal sequence consisting of the random signals is actually recorded in the optical recording medium to be inspected at Step S64 and the jitter thereof is measured at Step S65.

[0085] It is then judged at Step 66 whether the thus measured jitter is equal to or lower than a predetermined value. When it is found that the jitter exceeds the predetermined value, the inspection operation is returned to Step 63 and the recording conditions are reset. The predetermined value is preferably set to be equal to or lower than 9%, preferably 8%.

[0086] When it is judged at Step 66 that the jitter is equal to or lower than the predetermined value, a “constant-current laser beam” is then set at Step S67 for the inspection apparatus as a recording signal and recording conditions are set at Step 68 in such a manner that the recording linear speed is set to be the same as that set at Step 63 and that the power of the constant-current laser beam is changed step by step. It is necessary to set the interval between successive changes in the power of the constant-current laser beam so as to be sufficiently longer than a clock cycle and, for example, the power of the constant-current laser beam may be stepwise changed once each time the optical recording medium makes one revolution. When the setup has been completed, the constant-current laser beam whose power is controlled so as to be changed step by step is at Step 69 projected onto the random signal sequence recorded at Step S64.

[0087] A “random signal” is then set again for the inspection apparatus as a recording signal to be recorded in the optical recording medium at Step S70, and recording conditions are set at Step 71 to be the same as those set at Step 63. When the setup has been completed, the random signal sequence is at Step 72 again recorded on the random signal sequence recorded at Step 63 and jitter in respective areas irradiated with the constant-current laser beams having different powers are measured at Step S73.

[0088] It is then judged at Step 74 whether or not the jitter exceeds a level indicating that a reproduced signal contains error which cannot be corrected, concretely 13%, preferably 15%. When it is found that the jitter exceeds 13% or 15%, it is judged at Step 75 that the optical recording medium is a write-once type medium. To the contrary, when it is judged that the jitter does not exceed 13% or 15%, it is judged at Step 76 that the optical recording medium is a rewritable type medium.

[0089] Thus, it is possible to judge whether or not the optical recording medium meets the fourth condition.

[0090] It is unnecessary to effect inspection for judging whether or not the first to fourth conditions are met on all optical recording media to be shipped and it is possible to select at least one optical recording medium from each lot of optical recording media subjected to the layer forming step and the initialization step and inspect the selected optical medium in accordance with the above described inspection process, whereby it can be judged whether optical recording media included in the lot are write-once type media or rewritable type media. Therefore, the inspection process can be considered to constitute a part of the manufacturing process of optical recording media and in this case, the manufacturing process of optical recording media is performed in the order of the layer forming process, the initialization process and the inspection process.

[0091] In a driving apparatus for optical recording media, a high frequency wave having a frequency an order of magnitude higher than the recording frequency, such as a high frequency wave having a frequency of several 100 MHz, is generally superposed on a drive signal for modulating intensities of a laser beam for recording data, a laser beam for reading data and a laser beam for erasing data. The constant-current laser beam includes a laser beam driven by a direct current on which such a high frequency wave is superposed.

[0092] Hereinafter, one example of an optical recording medium to be inspected in accordance with the inspection method according to the present invention will be explained.

[0093] In the present invention, it is preferable to optimize the heat design of an optical recording medium and the composition of the recording layer so that the initialization can be made at a relatively high linear speed and recording marks cannot be erased at a relatively low linear speed. Concretely, it is preferable for the recording layer to have configuration which can be relatively quickly cooled, namely, a rapid cooling configuration such that amorphous record marks are resistant to recrystallization. In the case where initialization is conducted by a bulk eraser using a laser beam having a large diameter, unlike the case where a laser beam having a small diameter is used, since the cooling rate of the recording layer is not so increased even if it has the rapid cooling configuration, the linear speed for initialization is not so decreased. Therefore, if the medium is designed so as to have the rapid cooling configuration, initialization can be made at a relatively high linear speed and data can be recorded at a relatively low linear speed. The media can be made so as to have the rapid cooling configuration by providing a metal reflective layer as a heat radiation layer on the recording layer via a dielectric layer, making the dielectric layer thin so that heat produced in the recording layer is quickly transmitted to a reflective layer or increasing the thermal conductivity of the dielectric layer and/or the thermal conductivity of the reflective layer.

[0094] In the present invention, the recording linear speed is not particularly limited. However, since it is necessary to set the initialization linear speed to be lower than the recording linear speed, it is necessary to set the initialization linear speed to be much lower in order to initialize the recording layer which is optimized for recording data at a low recording linear speed and the productivity of the optical recording media has to be lowered. Further, a recording layer which is optimized for recording data at a low recording linear speed sometimes cannot be initialized. On the other hand, if the recording linear speed is set high, it becomes difficult to suppress field deflection within the tolerance range and vibration of the motor for driving the medium becomes large unless the optical recording medium is fabricated with high mechanical accuracy, whereby it becomes difficult to record data stably. For these reasons, it is ordinarily preferable to set the recording linear speed to be 2 to 20 m/sec, preferably 3 to 15 m/sec.

[0095] Further, in the present invention, it is preferable to provide a recording layer which can be relatively easily crystallized and diffuse elements capable of preventing the recording layer from being crystallized from a dielectric layer adjacent to the recording layer into the recording layer at initialization. By this, it is possible to achieve an optical recording medium which can be initialized at a relatively high linear speed and in which recording marks resist recrystallization. In other words, it is possible to achieve a phase change type medium which can be used as a write-once type medium and can be easily initialized. In order to fabricate such an optical recording medium, it is preferable to provide a recording layer containing at least Sb and Te and provide a dielectric layer that contains S (sulfur) and is adjacent to the recording layer.

[0096] The configuration of one example of an optical recording medium according to the present invention will now be explained.

[0097] Configuration of an Optical Recording Medium Shown in FIG. 5

[0098]FIG. 5 shows one example of an optical recording medium according to the present invention. The optical recording medium includes a first dielectric layer 31, a recording layer 4, a second dielectric layer 32, a reflective layer 5 and a protective layer 6 on a translucent substrate 2 in this order and a laser beam for recording data or reading data is projected through the translucent substrate 2.

[0099] Translucent Substrate 2

[0100] The translucent substrate 2 has a property of transmitting a laser beam for recording data or reading data. The translucent substrate 2 has a thickness of 0.2 to 1.2 mm, preferably 0.4 to 1.2 mm. The translucent substrate 2 is normally formed of resin but may be formed of glass. Groove (guide grooves) 21 normally formed in an optical recording medium are disposed on the incident side of a laser beam and lands 22 are formed between neighboring grooves to be convex.

[0101] In the present invention, the grooves 21 and lands 22 can be used as a recording track.

[0102] First Dielectric Layer 31 and Second Dielectric Layer 32

[0103] These dielectric layers serve to prevent oxidization and deterioration of the recording layer 4 and block heat transmitted from the recording layer when data are recorded or radiate it along them, thereby protecting the support substrate 2 and the recording layer 4. Further, the degree of modulation of the laser beam can be improved by providing these dielectric layers. Each of the first and second dielectric layers may be formed by laminating two or more layers having different compositions.

[0104] Illustrative examples of dielectric materials used for these dielectric layers include preferable compounds containing at least one metal component selected from the group consisting of Si, Ge, Zn, Al, rare earth elements and the like. An oxide, nitride or sulfide is preferable as the compound and a mixture of two or more of them may be used.

[0105] As explained above, in the present invention, it is preferable for the dielectric layer disposed adjacent to the recording layer to contain elements capable of preventing the recording layer from being crystallized and to set the initialization condition in such a manner that the elements diffuse into the recording layer at initialization. S is preferable as the element capable of preventing the recording layer from being crystallized. Therefore, it is preferable in the present invention for one of the first dielectric layer 31 and the second dielectric layer 32, particularly, the first dielectric layer 31, to contain sulfide. Zinc sulfide (ZnS) is preferable as sulfide because a high refractive index can be obtained. However, if the dielectric layer is formed of ZnS alone, the stress of the dielectric layer becomes too high and, therefore, a mixture of ZnS and SiO₂ (ZnS—SiO₂) is preferably used.

[0106] Further, as explained above, in the present invention, the medium is preferably constituted so as to have a rapid cooling configuration, and for this purpose the dielectric layers, particularly, the second dielectric layer 32 is preferably formed of dielectric material having a high heat conductivity. Illustrative examples of preferable dielectric materials having high heat conductivities include the mixture of zinc sulfide and silicon oxide (ZnS—SiO₂), aluminum nitride, aluminum oxide, silicon nitride, tantalum oxide and the like and oxide and/or nitride of aluminum and oxide and/or nitride of silicon are particularly preferable. The ZnS—SiO₂ preferably contains 30 to 60 mol of SiO₂. If the amount of SiO₂ is too small, heat conductivity of the dielectric layer becomes too low. On the other hand, if the amount of SiO₂ is too great, since adhesiveness with other layers becomes lower, the dielectric layer tends to exfoliate from other layers during long-term storage of the optical recording medium.

[0107] The second dielectric layer preferably has a heat conductivity of 1 W/mK or more, more preferably 1.5 W/mK or more. The upper limit of the heat conductivity of the second dielectric layer is not particularly limited but heat conductivities of the materials usable for the dielectric layer are normally equal to or lower than 20 W/mK. In this specification, the heat conductivity of the second dielectric layer is defined not as a value measured in a thin film state but a value measured in a bulk state.

[0108] The thicknesses of the first dielectric layer and the second dielectric layer may be determined so as to ensure protection ability and improvement in the degree of modulation of the laser beam. Normally, the first dielectric layer 31 has a thickness of 30 to 300 nm, preferably 50 to 250 nm and the second dielectric layer 32 has a thickness of 10 to 50 nm. However, in order to constitute an optical recording medium to have a rapid cooling configuration, the second dielectric layer 32 preferably has a thickness equal to or thinner than 30 nm, more preferably equal to or thinner than 25 nm.

[0109] The respective dielectric layers are preferably formed using a sputtering process.

[0110] Recording Layer 4

[0111] The composition of the recording layer is not particularly limited. The recording layer may be formed of materials selected from various phase change materials but preferably contains at least Sb and Te. If the recording layer is formed of only Sb and Te, since the crystallization temperature thereof is low, about 130 degrees, and long term storage reliability is low, it is preferable to add other elements to the recording layer in order to increase the crystallization temperature thereof. As an element to be added to the recording layer, it is preferable to use at least one element selected from the group consisting of In, Ag, Au, Bi, Se, Al, P, Ge, H, Si, C, V, W, Ta, Zn, Ti, Sn, Pb, Pd and rare earth elements (Sc, Y and lanthanoids). Among these elements, it is most preferable to use at least one element selected from the group consisting of rare earth elements, Ag, In and Ge because the long term storage reliability can be particularly improved.

[0112] When Sb and Te are contained, the recording layer preferably has a composition expressed by the following chemical formula:

(Sb_(x)Te_(1-x))_(1-y)M_(y)

[0113] wherein M is an element other than Sb and Te, x is equal to or larger than 0.2 and equal to or smaller than 0.9 and y is equal to or larger than 0 and equal to or smaller than 0.4. It is preferable that x is equal to or larger than 0.5 and equal to or smaller than 0.7 and y is equal to or larger than 0.01 and equal to or smaller than 0.2. If x indicating the amount of Sb is too small, the crystallization rate becomes too low and it becomes difficult to initialize the recording layer. Further, since reflective coefficients of crystalline areas of the recording layer become low, the reproduction output becomes low. Furthermore, if x is extremely low, data recording itself becomes impossible. On the other hand, if x is too large, since the crystallization rate becomes too high, the recording layer is not suitable for that of a write-once type medium. Further, if x is too large, since the difference in reflective coefficients between a crystalline state and an amorphous state becomes small, the reproduction output becomes low. Therefore, it is possible to obtain a crystallization rate of the recording layer suitable for that of a write-once type medium and increase the reproduction output by determining x within the above defined range. The concrete value of x may be determined in accordance with the recording linear speed.

[0114] The element M is not particularly limited but is preferably selected from among the above defined elements capable of improving the long term storage reliability. If y indicating the amount of the element M is too large, the crystallization rate of the recording layer becomes too high and the reproduction output becomes low.

[0115] The recording layer is preferably thicker than 4 nm and equal to or thinner than 50 nm, more preferably equal to or thicker than 5 nm and equal to or thinner than 30 nm. If the recording layer is too thin, crystalline phase is hard to grow and the recording layer is hard to crystallize. On the other hand, if the recording layer is too thick, since the heat capacity of the recording layer becomes large, it becomes difficult to record data and the reproduction output becomes low.

[0116] The recording layer is preferably formed using a sputtering process.

[0117] Further, in the present invention, the structure of the recording layer is not particularly limited. For example, the present invention is applicable to the medium having multiple recording layers described in Japanese Patent Application Laid Open No. 8-221814 or Japanese Patent Application Laid Open No. 10-226173.

[0118] Reflective Layer 5

[0119] In the present invention, the material used for forming the reflective layer is not particularly limited and the reflective layer is normally formed of a metal such as Al, Au, Ag, Pt, Cu, Ni, Cr, Ti, Si and the like or metalloid alone or alloy containing at least one of these. As explained above, since it is preferable for the optical recording medium to have a rapid cooling configuration, the reflective layer is preferably formed of materials having high heat conductivity. As a material having high heat conductivity, Ag or Al is preferable for forming the reflective layer. However, since a reflective layer having high corrosion resistance cannot be formed using Ag or Al alone, another element is added to the reflective layer for improving corrosion resistance.

[0120] However, when another element is added to the reflective layer, since the heat conductivity of the reflective layer is lowered, it is preferable to employ Ag having a higher heat conductivity as a primary component for the reflective layer in the case where another element is added thereto. Illustrative examples of elements added to the reflective layer as sub-component together with Ag include at least one element selected from the group consisting of Mg, Pd, Ce, Cu, Ge, La, S, Sb, Si, Te and Zr. Among these, at least one of element, preferably two or more elements are added to the reflective layer as sub-component elements. The amount of each of the sub-component elements contained in the reflective layer is preferably 0.05 to 2.0 atomic %, more preferably 0.2 to 1.0 atomic % and the amount of all sub-component elements contained in the reflective layer is preferably 0.2 to 5 atomic %, more preferably 0.5 to 3 atomic %. If the amount of the sub-component elements is too small, the above effect cannot be obtained. On the contrary, if the amount of the sub-component elements is too large, the heat conductivity of the reflective layer becomes high.

[0121] The heat conductivity of the reflective layer is preferably equal to or higher than 100 W/mK, more preferably equal to or higher than 150 W/mK. The heat conductivity of the reflective layer can be calculated, for example, by the Widemann-Franz principle based on the electric resistance value obtained by the four probe method of the resistivity measurement. The upper limit of heat conductivity of the reflective layer is not particularly limited. Therefore, pure silver, whose heat conductivity, 250 W/mK, is highest among materials usable for the reflective layer, can be used for forming the reflective layer.

[0122] The reflective layer is preferably formed to have a thickness of 10 to 300 nm. If the reflective layer is thinner than 10 nm, the reflective coefficient of the reflective layer becomes low and, on the other hand, even if the reflective layer is thicker than 300 nm, the increase in the reflective coefficient is small and cost is increased. The reflective layer is preferably formed using a gas phase growth process such as sputtering, vacuum deposition or the like.

[0123] Protective Layer 6

[0124] The protective layer 6 is formed for improving resistance to scuffing and corrosion resistance. It is preferable to form the protective layer of various organic substances and particularly preferable to form the protective layer of substances obtained by curing radiation curable compound or the constituents thereof with radiation such as an electron beam, ultraviolet rays or the like. The protective layer normally has a thickness of 0.1 to 100 μm and is normally formed using a spin coating method, a gravure coating method, a spray coating method, a dip coating method or the like.

[0125] Configuration of an Optical Recording Medium Shown in FIG. 6

[0126]FIG. 6 shows another example of an optical recording medium according to the present invention. The optical recording medium is obtained by layering a reflective layer 5 formed of metal or metalloid, a second dielectric layer 32, a recording layer 4, a first recording layer 32 and a translucent substrate 2 on a support substrate 20 in this order. A laser beam for recording data or reading data is projected through the translucent substrate 2. An intermediate layer containing a dielectric material may be formed between the support substrate 2 and the reflective layer 5.

[0127] As the translucent substrate 2 in this example, there can be used a resin plate or a glass plate having substantially the same thickness as that of the translucent substrate 2 shown in FIG. 5. However, it is preferable to thin the translucent substrate 2 so that data can be recorded with high density by increasing NA of a recording and reading optical system. In this case, the thickness of the translucent substrate is preferably 30 to 300 μm. If the translucent substrate is too thin, the optical influence of dust adhered to the surface of the translucent substrate is increased. On the other hand, if the translucent substrate is too thick, it becomes difficult to record data with high density even if the NA is increased.

[0128] The translucent substrate can be made thinner by, for example, adhering a translucent sheet made of translucent resin to the first dielectric layer 31 to form the translucent substrate or directly forming a translucent resin layer on the first dielectric layer 31 using a coating method to form the translucent substrate.

[0129] The support substrate 20 is provided for ensuring rigidity of the optical recording medium. The thickness of the support substrate 20 and the material for forming the support substrate 20 can be selected similarly to the translucent substrate 2 shown in FIG. 5. The support substrate 20 may be transparent or opaque. As illustrated, grooves can be formed by transferring grooves formed on the support substrate 20 onto the respective layers to be formed thereon.

[0130] In the medium shown in FIG. 6, the surface roughness of the reflective layer on the laser beam entering side tends to be increased by crystal growth when the reflective layer is formed. When the surface roughness of the reflective layer is increased, noise in reproduced signals is increased. Therefore, it is preferable to reduce the crystal size of the reflective layer and form the reflective layer as an amorphous layer. For these surfaces, it is preferable for the reflective layer to contain Ag or Al as a primary component and the above specified sub-component elements.

[0131] Since the heat conductivity of the reflective layer increases as the crystal size is smaller, when the reflective layer is amorphous, it is difficult to obtain a high cooling rate when data are recorded. Therefore, it is preferable to once form the reflective layer as an amorphous layer and heat the reflective layer to be crystallized. When the reflective layer once formed as an amorphous layer is crystallized, the surface roughness of the reflective layer which was the amorphous layer can be substantially maintained and the heat conductivity of the reflective layer can be increased by crystallization.

[0132] Other layers are the same as those of the medium shown in FIG. 5.

WORKING EXAMPLES AND COMPARATIVE EXAMPLES Working Example 1

[0133] An optical recording medium sample No. 1 having the configuration shown in FIG. 5 was fabricated by employing a disk-like polycarbonate substrate having a diameter of 120 mm and a thickness of 0.6 mm and simultaneously formed by injection molding with grooves having a width of 0.2 μm, depth of 20 nm and pitch of 0.74 μm as the translucent substrate 2 and forming the first dielectric layer 31, recording layer 4, second dielectric layer 32, reflective layer 5 and protective layer 6 on the polycarbonate substrate in this order in the following manner.

[0134] The first recording layer 31 was formed in an atmosphere containing Ar gas by sputtering using a target of 80 mol % of ZnS and 20 mol % of SiO₂. The thickness of the first recording layer 31 was 90 nm.

[0135] The recording layer 4 was formed in atmosphere containing Ar gas by sputtering. The composition (atomic ratio) of the recording layer 4 was (Sb_(0.67)Te_(0.33))_(0.9)In_(0.04)Ag_(0.05). The thickness of the recording layer 4 was 20 nm.

[0136] The second recording layer 32 was formed in an atmosphere containing Ar gas by sputtering using a target of ZnS (50 mol %)-SiO₂ (50 mol %). The thickness of the first recording layer 32 was 20 nm. The heat conductivity of ZnS (50 mol %)-SiO₂ (50 mol %) used as a target was 1.0 W/mK.

[0137] The reflective layer 5 was formed in an atmosphere containing Ar gas by sputtering using a target of alloy of Al-1.7 mol % Cr. The thickness of the reflective layer 5 was 100 nm. The heat conductivity of the reflective layer 5 was 40 W/mK.

[0138] The protective layer 6 was formed by coating ultraviolet curable resin by a spin coating method and curing it by irradiation with ultraviolet rays. The thickness of the protective layer 6 after curing was 5 μm.

[0139] The thus fabricated sample No. 1 was initialized. The initialization was performed using a bulk eraser at a linear speed of 2 m/sec.

[0140] Shortest signals and random signals were recorded once in the sample No. 1 using a DDU1000 optical recording medium evaluation apparatus manufactured by Pulstec Industrial Co., Ltd. under the following conditions:

[0141] Wavelength X of laser beam: 635 nm

[0142] Numerical aperture NA of objective lens: 0.6

[0143] Signal to be recorded: single signal (3T signal which is the shortest signal) by EFM plus (8-16) modulation and random signals

[0144] Linear speed: 3.5 m/sec (the shortest signal length: 0.38 λ/NA)

[0145] Thereafter, recorded data were reproduced using a reproduction power of 0.9 mW.

[0146] The pattern of modulating the laser beam for recording was as follows. When data are to be recorded in a phase change type optical recording medium, a recording beam is generally not projected in the form of a direct current corresponding to the length of a record mark but is projected in the form of multi-pulses as described in Japanese Patent Application Laid Open No. 2000-155945. One example of recording waveform in the form of multiple-pulses is shown in FIG. 7. In this specification, the recording waveform means the pattern of a drive signal for modulating the intensity of the recording beam. FIG. 7 shows the 5T signal of an NRZI signal and a recording waveform corresponding to the 5T signal. In FIG. 7, Pw designates recording power and Pb designates bias power. Pb is normally referred to as erasing power in a recording system in which data can be overwritten. The recording waveform includes recording pulse portions and direct current portions each connecting neighboring recording pulse portions. The recording pulse portion is constituted by repeating a combination of an upward pulse (intensity: Pw) and a following downward pulse (intensity: Pb). As a whole, the recording pulse portion is raised from Pb and returned to Pb. In FIG. 7, Ttop is the width of the foremost upward pulse and Tmp is the widths of other upward pulses (which may be referred to as multi-pulses). The widths of these pulses are normalized by the reference clock width (1T).

[0147] In Working Example 1, data were recorded using such a recording waveform under the following conditions.

[0148] Recording power Pw: 9 mW

[0149] Bias power Pb: 0.5 mW

[0150] Ttop: 0.6 T

[0151] Tmp: 0.35 T

[0152] In the shortest signal (3T signal), there was one upward pulse in each of the recording pulse portions and the width thereof was Ttop. These recording conditions corresponded to the optimum recording conditions for minimizing jitter.

[0153] After data were recorded, the CNR of the shortest signal was measured using a Spectrum Analyzer manufactured by ADVANTEST CORPORATION and was found to be 49.1 dB. Further, clock jitter and reproduction output of the random signal were measured and were found to be 8.5% and 1.04 volt, respectively. When the clock jitter is equal to or lower than 9%, signals can be reproduced and there is no practical problem. On the other hand, when the clock jitter exceeds 13%, particularly when it exceeds 15%, since error frequently occurs, the reproduced signal cannot be utilized. The clock jitter was obtained by measuring reproduced signals using a Time Interval Analyzer manufactured by YOKOKAWA ELECTRIC CORPORATION to obtain the fluctuation a of signals and calculating σ/Tw (%), wherein Tw was the width of the detection window. The reproduction output was measured using an analog oscilloscope.

[0154] The track on which signals were recorded was then irradiated with a constant-current laser beam having a power of 2 to 7 mW at a linear speed of 3.5 m/sec in order to erase the record marks. Thereafter, CNR was measured and the maximum signal attenuation was found to be 16.5 dB at the areas of the track on which the shortest signal were recorded. On the other hand, the ratio of the reproduction output after the irradiation with the constant-current laser beam to the reproduction output before the irradiation was 0.29 at the areas of the track on which the random signals were recorded.

[0155] Random signals were then recorded on the track irradiated with the constant-current laser beam at a linear speed of 3.5 m/sec and clock jitter thereof was measured. It was found that the minimum value was 16.8% and the reproduced signal could not be utilized.

[0156] In this sample No. 1, since the ratio of erasure was low when the constant-current laser beam was projected, the clock jitter was extremely high when random signals were recorded after the irradiation with the constant-current laser beam. Therefore, it was found that it was impossible to rewrite data in this sample No. 1.

[0157] When the power of the constant-current laser beam was increased to higher than 7 mW, the recording layer was melted and became amorphous after the irradiation with the laser beam. Namely, it was found that even if the power of the laser beam was increased, it was impossible to re-crystallize the recording layer.

[0158] After the initialization, random signals were recorded under the above defined conditions on the areas in which the random signals had been recorded under the above defined conditions and clock jitter was then measured. It was found that the clock jitter exceeded 20% and could not be measured, and it was impossible to reproduce signals.

[0159] After data were recorded under the above defined conditions, the recording layer of the sample No. 1 was observed using a transmission electron microscope. As shown in FIG. 8, it was found that the shortest record mark was substantially circular.

COMPARATIVE EXAMPLE

[0160] A sample No. 2 was fabricated in the manner of Working Example 1 except that the first dielectric layer had a thickness of 120 nm and the second dielectric layer 32 had a thickness of 50 nm.

[0161] Shortest signals and random signals were recorded in the sample No. 2 in the manner of Working Example 1 except that the recording power Pw was set to be 8 mW, and the CNR, reproduction output and jitter were measured. The CNR of the shortest signal was 49 dB, the reproduction output of the random signals was 0.98 volts and the jitter was 10%. Therefore, it was found that the CNR was substantially the same as that of Working Example 1 but jitter was increased in comparison with Working Example 1.

[0162] The sample No. 2 was irradiated with the constant-current laser beam similarly to Working Example 1, and it was found that the carrier attenuation of the shortest signal was 18 dB. Further, signals were again recorded in the sample No. 2 after the irradiation with the constant-current laser beam, and it was found that clock jitter of the signals again recorded exceeded 15%.

[0163] When recording layer of the sample No. 2 was observed using a transmission electron microscope, it was found that the shortest record mark was not circular but the rear end edge thereof was scooped out and a portion in the vicinity of the rear end edge was re-crystallized. It is reasonable to consider that this re-crystallization occurred because this sample No. 2 was fabricated so as to have gradual cooling configuration by thickening the second dielectric layer in comparison with Working Example 1. Further, it is reasonable to consider that the jitter became worse because the degree of re-crystallization varied greatly among the recording marks.

[0164] Comparison of a Write-once Type Medium and a Rewritable Type Medium

[0165] A sample No. 3 was fabricated in the manner of the sample No. 1 in Working Example 1 except that the composition of the recording layer was (Sb_(0.7)Te_(0.3))_(0.9)In_(0.04)Ag_(0.05). Since the sample No. 3 had a recording layer whose crystallization rate was higher than that of Working Example 1, data could be rewritten at 3.5 m/sec in the sample No. 3.

[0166] Random signals were recorded in each of the sample No. 1 and the sample No. 3 similarly to Working Example 1 and Rini was measured. Each of the samples was then irradiated with the constant-current laser beam at the same linear speed as that for recording signals, thereby conducting an erasing operation. The relationship between the power of the projected constant-current laser beam (DC erasing power) and the linear speed V are shown in FIG. 9. Rtop and Rbottom were then measured and (Rtop +Rbottom)/2Rini was calculated. The results are shown in FIG. 9. The reflective levels were measured using a DDU1000 optical recording medium evaluation apparatus manufactured by Pulstec Industrial Co., Ltd.

[0167] As shown in FIG. 9, (Rtop+Rbottom)/2Rini was equal to or larger than 1 in the sample No. 3 when the linear speed was 3.5 m/sec and the power of the constant-current laser beam was 3 mW but (Rtop+Rbottom)/2Rini was smaller than 1 in the sample No. 1 irrespective of the power of the constant-current laser beam when the linear speed was equal to or higher than 3.5 m/sec. Further, after the constant-current laser beam was projected, random signals were again recorded under the same conditions as that for the first recording in each of the sample No. 1 and the sample No. 3 and clock jitter was measured. When the power of the constant-current laser beam was equal to or higher than 3 mW, the clock jitter was 8 to 9% in the sample No. 3 and the clock jitter was 16.8 to 20% in the sample No. 1. Therefore, it was found that data could be rewritten in the sample No. 3 at the linear speed of 3.5 m/sec and that data could not be rewritten at the linear speed of 3.5 to 14 m/sec.

[0168] According to the present invention, it is possible to manufacture a write-once type and phase change type medium which can lower jitter and achieve excellent high reproduction output even when data are recorded at high density.

[0169] Further, according to the present invention, it is possible to correctly judge whether a phase change type medium can be used as a write-once type medium. 

1. A method for inspecting an optical recording medium comprising steps of recording a shortest signal in an optical recording medium having a phase change type recording layer so that CNR of the shortest signal is equal to or higher than 45 dB, irradiating an area in which the shortest signal was recorded with a constant-current laser beam whose power level cannot melt the recording layer at the same linear speed as that at which the shortest signal was recorded, measuring decrease in the carrier of the shortest signal and judging the optical recording medium to be a write-once type medium when the decrease in the carrier is equal to or lower than 20 dB.
 2. A method for inspecting an optical recording medium in accordance with claim 1, wherein the shortest signal is recorded so that the CNR is equal to or higher than 48 dB.
 3. A method for inspecting an optical recording medium in accordance with claim 1 or 2, wherein the optical recording medium is judged to be a write-once medium when the decrease in the carrier is equal to or lower than 18 dB.
 4. A method for inspecting an optical recording medium in accordance with claim 3, wherein the optical recording medium is judged to be a write-once medium when the decrease in the carrier is equal to or higher than 5 dB.
 5. A method for inspecting an optical recording medium in accordance with claim 4, wherein the optical recording medium is judged to be a write-once medium when the decrease in the carrier is equal to or higher than 10 dB.
 6. A method for inspecting an optical recording medium comprising the steps of recording random signals in the optical recording medium having a phase change type recording layer, conducting reproducing operation of the signals on an area in which the random signals were recorded, thereby measuring a highest reflective level Rini, irradiating the area in which the random signals were recorded with a constant-current laser beam at the same linear speed as that at which the random signals were recorded, conducting reproducing operation of the signals on the area irradiated with the constant-current laser beam, thereby measuring a highest reflective level Rtop and a lowest reflective level Rbottom, and judging the optical recording medium to be a write-once type medium when (Rtop+Rbottom)/2Rini<1 is satisfied, irrespective of whether the constant-current laser beam has a power level which can melt the recording layer.
 7. A method for inspecting an optical recording medium in accordance with claim
 6. wherein the optical recording medium is judged to be a write-once medium when (Rtop+Rbottom)/2Rini<0.95 is satisfied.
 8. A method for inspecting an optical recording medium comprising the steps of recording random signals in the optical recording medium having a phase change type recording layer, again recording the random signals so as to be superposed on the first mentioned random signals at the same linear speed as that at which the first mentioned random signals were recorded, conducting reproducing operation of the signals, and judging the optical recording medium to be a write-once type medium when the signals cannot be reproduced.
 9. A method for inspecting an optical recording medium in accordance with claim 8, wherein it is judged that the signals cannot be reproduced when the reproduced signal contains error which cannot be corrected.
 10. A method for inspecting an optical recording medium in accordance with claim 9, wherein it is judged that the signals cannot be reproduced when clock jitter exceeds 13%.
 11. A method for inspecting an optical recording medium in accordance with claim 10, wherein it is judged that the signals cannot be reproduced when clock jitter exceeds 15%.
 12. A method for inspecting an optical recording medium comprising the steps of recording random signals in the optical recording medium having a phase change type recording layer, irradiating an area in which the random signals were recorded with a constant-current laser beam at the same linear speed as that at which the random signals were recorded, thereby again recording random signals in the area irradiated with the constant-current laser beam, conducting reproducing operation of the signals, and judging the optical recording medium to be a write-once type medium when the signals cannot be reproduced.
 13. A method for inspecting an optical recording medium in accordance with claim 12, wherein it is judged that the signals cannot be reproduced when the reproduced signal contains error which cannot be corrected.
 14. A method for inspecting an optical recording medium in accordance with claim 13, wherein it is judged that the signals cannot be reproduced when clock jitter exceeds 13%.
 15. A method for inspecting an optical recording medium in accordance with claim 14, wherein it is judged that the signals cannot be reproduced when clock jitter exceeds 15%.
 16. A method for manufacturing an optical recording medium comprising a layer forming step of forming at least a phase change recording layer, an initialization step of crystallizing at least an area of the recording layer in which data are to be recorded and an inspection step of judging whether the optical recording medium subjected to the initialization step is a write-once type medium or a rewritable type medium, the inspecting step comprising the steps of recording a shortest signal in an optical recording medium having a phase change type recording layer so that CNR of the shortest signal is equal to or higher than 45 dB, irradiating an area in which the shortest signal was recorded with a constant-current laser beam whose power level cannot melt the recording layer at the same linear speed as that at which the shortest signal was recorded, measuring decrease in the carrier of the shortest signal and judging the optical recording medium to be a write-once type medium when the decrease in the carrier is equal to or lower than 20 dB.
 17. A method for manufacturing an optical recording medium comprising a layer forming step of forming at least a phase change recording layer, an initialization step of crystallizing at least an area of the recording layer in which data are to be recorded and an inspection step of judging whether the optical recording medium subjected to the initialization step is a write-once type medium or a rewritable type medium, the inspecting step comprising the steps of recording random signals in the optical recording medium having a phase change type recording layer, conducting reproducing operation of the signals on an area in which the random signals were recorded, thereby measuring a highest reflective level Rini, irradiating the area in which the random signals were recorded with a constant-current laser beam at the same linear speed as that at which the random signals were recorded, conducting reproducing operation of the signals on the area irradiated with the constant-current laser beam, thereby measuring a highest reflective level Rtop and a lowest reflective level Rbottom, and judging the optical recording medium to be a write-once type medium when (Rtop+Rbottom)/2Rini<1 is satisfied, irrespective of whether the constant-current laser beam has a power level which can melt the recording layer.
 18. A method for manufacturing an optical recording medium comprising a layer forming step of forming at least a phase change recording layer, an initialization step of crystallizing at least an area of the recording layer in which data are to be recorded and an inspection step of judging whether the optical recording medium subjected to the initialization step is a write-once type medium or a rewritable type medium, the inspecting step comprising the steps of recording random signals in the optical recording medium having a phase change type recording layer, again recording the random signals so as to be superposed on the first mentioned random signals at the same linear speed as that at which the first mentioned random signals were recorded, conducting reproducing operation of the signals, and judging the optical recording medium to be a write-once type medium when the signals cannot be reproduced.
 19. A method for manufacturing an optical recording medium comprising a layer forming step of forming at least a phase change recording layer, an initialization step of crystallizing at least an area of the recording layer in which data are to be recorded and an inspection step of judging whether the optical recording medium subjected to the initialization step is a write-once type medium or a rewritable type medium, the inspecting step comprising the steps of recording random signals in the optical recording medium having a phase change type recording layer, irradiating an area in which the random signals were recorded with a constant-current laser beam at the same linear speed as that at which the random signals were recorded, thereby again recording random signals in the area irradiated with the constant-current laser beam, conducting reproducing operation of the signals, and judging the optical recording medium to be a write-once type medium when the signals cannot be reproduced. 